Dan Mendels, Ariel Ben Sasson , Michael Greenman , Nir Yaacobi-Gross, Ariel Epstein, Nir Tessler

1. Electrical Engineering Perspective of a Material Driven Field Dan Mendels, Ariel Ben Sasson, Michael Greenman, Nir Yaacobi-Gross, Ariel Epstein, Nir Tessler Sara & Moshe Zisapel Nanoelectronic Center Electrical Engineering Dept. Haifa 32000 Israel www.ee.technion.ac.il/nir

2. Outline • Transport (do we know it all?) • Material manipulations by EE • Engineering tool for OLED analysis • Engineering FET structure (VOFET)

3. Technion Campus

4. Lab to FAB Large size companies got seriously involved at around 1997 Today, 15 years later, there are still some basics we don’t know In the context of amorphous organic semiconductors

5. Trivia • Chemical potential • Fermi level • Equilibrium • Drift Diffusion Warning: each topic is dealt by at least two different communities (disciplines) that have developed their own “obvious” meaning of the terminology.

6. Chemical Potential E Eg Equilibrium Vacuum Level Conduction Ec Ev Valence Chemical potential is the energy to be added to the system if an electron is added. Electron can reside either in Valence or Conduction band according to Fermi Dirac statistics. Under equilibrium it equals the parameter in the FD statistics (EF). We consider the following reaction:

7. Quasi Chemical Potential E E Non-Equilibrium Vacuum Level Vacuum Level Ec Ec • Generally: • Quasi Chemical potential • ≠ • EFin the FD statistics (quasi - EF). • (for ideal gas they are equal) QCPe QCPh Ev Ev Conduction EFe EFh Valence Quasi Chemical potential is the energy to be added to the system if an electron is added. Electron can reside only in the band the quasi chemical potential refers to. In the respective band it will reside according to Fermi Dirac statistics with the respective quasi - EF.

8. Disorder and Anderson Localization E E High Order Band x Density of states Low disorder E E Band Tail states (traps) x Density of states High disorder E E Density of localized states x Density of states

9. Transport R R DE DE InP InGaAs PFOBT PFO When considering many molecules or sites one uses a variant of Mott’s Variable Range Hopping

10. Mott’s Variable Range Hopping r and DE are determined so as to maximize the hopping rate For a constant density of states: For a shaped density of states: e E Transport Energy (Et=?) Effective intermediate energy DE Effective initial energy r

11. Transport Energy KBT J. O. Oelerich et. al. APL, 97, 143302, 2010. Effective Initial Energy (QCP) D. Mendels & N. Tessler, Submitted.

12. 1. Mobility is charge density dependent KBT 2. Transport Energy 3. is Effective Initial Energy There is transport of energy even in the absence of Temperature gradients (a)

13. If Seebeck effect The effect is linked to charge density gradients. Let’s examine a case where current flows and there is no charge gradient D/m = Einstein relation BUT For most practical scenarios the GER is a very good approximation G.E.R. Monte-Carlo new formalism

14. Engineers’ Engineering of Materials Creating building blocksto allow for device engineering

15. Building Electronic Materials with LEGO bricks

16. Electronic Peptides The basic idea Of the shelf peptide synthesizer O R1 R2 O Electronic-grade Tailor-made The materials of organic displays The materials of Nature Electronic Peptide YoavEichen, Chemistry, Technion

17. Works like a charm but for: Amide bond is only slightly conjugated Film morphology is difficult to control (probably H bonds) N. Tessler et. al., "Conjugated Polymer Electronics – Engineering Materials and Devices," in Handbook of Conducting Polymers, CRC, 2006

18. core • Colloidal nanocrystals (NCs) Organic capping layer (Ligands) • 8-18 aliphatic carbon chain • Allows solubility • Stabilizes the QDs • Act as a barrier for charge transfer Greenham, N. et al. Phys Rev B, 1996, 54, 17628

19. Ligands Exchange Free dipole Y NH2 SH X OCH3 CH3 NO2 Y Y Y Y Y Y Y InAs X X X X X X X TOP (trioctylphosphine) Michal Soreni-Harari, et. al., Nano Lett. 8, 678-684, (2008).

20. HOMO level shifting(Differential Pulsed Voltammetry) TOP (original) SH NH2 H CH3 MTP aniline InAs 4.4 nm Surface ligands Vacuum level -4.7eV -4.95eV -5.05eV

21. Size Matters Sub 2nm diameter 4.4nm diameter Why ?

22. Partial Ligands Exchange SH CH3 CH2(CH2)14CH3 CdSe Are we polarizing N. Yaacobi-Gross et. al. , "Molecular control of quantum-dot internal electric field …." Nat Mater, vol. 10, pp. 974-979, 2011. NH2

23. Adding small fractions of free MTP ligands. Mixed Ligands CdSe CH3 Mixed Ligands  Stark Shift SH HDA CdSe NH2 Nir Yaacobi-Gross et. al., Nature Materials 10, 974-979 doi:10.1038/nmat3133

24. Ligands induced bulk heterojunction in near IR active all nanocrystals solar cells Results: Stark shift helps to dissociate charges

25. Formulations Wide gap x-linkable conjugated matrix Triarylamine Emitter (green PPV) PVK-Cin Dopant (C60Fxx) m=3% n=48.5% o=48.5% Transport (P3HT) O. Solomeshch et. al, "Electronic formulations…” Adv. Func. Mat,16, 2095, 2006.

26. Formulations Patterning Stable Hole injection layer (PVK-Cin + P3HT + C60F36) or (PVK-Cin+ PolyTPD+C60F48) 110c in Vacuum Single pixel two-Color OLED (no loss of efficiency during processing) X-Linking  Stability O. Solomeshch et. al., "Electronic formulations - Photopatterning of luminescent conjugated polymers," Advanced Functional Materials, vol. 16, pp. 2095-2102, Oct 2006. O. Solomeshch, et. al., “…Electrical Doping of Fluorinated C-60 in Conjugated Polymers," Advanced Materials, vol. 21, pp. 4456-+, Nov 2009.

27. Engineering Tool Simple method to extract recombination zone parameters in OLEDs

28. Closed-form solution Image-Source Cathode ITO Spatial broadening factor Direct-Ray A. Epstein, N. Tessler, and P. D. Einziger, IEEE J. Quantum Elect., 46, 9, pp. 1388-1395 (2010). Weak-microcavity Spectral broadening

29. Emission zone (EZ) location effect • Very narrow emission zone • Different positions  Emission pattern extrema • Origin: Image-Source interference (reflecting cathode) Cathode ITO

30. Emission zone (EZ) width effect • Symmetrical distribution • Different widths  “Fringe visibility” (ratio) • Origin: Image-Source interference averaged Cathode ITO A. Epstein, N. Tessler, and P. D. Einziger, Opt. Lett.,35, 20, pp. 3366-3368 (2010).

31. Aluminium Blue P-OLED (CDT OLED) LEP 130nm IL 15nm HIL 35nm ITO 45nm • Device under test • Angular spectra Glass Substrate 0.7mm Glass hemisphere (centre is at emissive area) Angle [°] ‘p’ pol (TM) M. Roberts, K. Asada, M. Cass, C. Coward, S. King, A. Lee, M. Pintani, M. Ramon, and C. Foden, Proc. SPIE Photonics7722, 77220C, Brussels (2010) ‘s’ pol (TE)

32. Emission zone location • Maxima angles

33. Engineering Device Structures

34. O. Globerman, "Lateral And Vertical Organic Field Effect Transistors," M.Sc., Electrical Engineering, Technion Israel Institute of Technology, Israel, 2006. Device architecture 3D Illustration Macro scale top view 500μm 100μm Fundamental parameters 100μm Gate – Al Drain – Al L - Channel length - active layer thickness D - Perforations’ diameter (~80nm) FF - perforations area ratio td & ts Dielectric – td Active layer Patterned source – ts 100Å Au

35. How does it work? Drain Patterned source Gate Insulator Gate VG > 0

36. The effect of the perforations’ aspect ratio Measurements Simulations (a) (b) VDS=1V VDS=1V hS[nm] IG[A] 7 9 13 • φb0=0.68eV. S SC D e- Φb0 h+ C60 Al Au “Thick” source  Tunnel effect A. J. Ben-Sasson, E. Avnon, E. Ploshnik, O. Globerman, R. Shenhar, G. L. Frey, and N. Tessler, "Patterned electrode vertical field effect transistor fabricated using block copolymer nanotemplates," Applied Physics Letters, vol. 95, p. 213301, 2009. A. J. Ben-Sasson and N. Tessler, "Patterned electrode vertical field effect transistor: Theory and experiment," Journal of Applied Physics, vol. 110, p. 12, Aug 2011. A. J. Ben-Sasson, Z. Chen, A. Facchetti, and N. Tessler, "Solution-processed ambipolar vertical organic field effect transistor," Applied Physics Letters, vol. 100, pp. 263306-4, 2012. hS D=80nm ON Drain Gate Insulator Gate

37. “Fast” Switching (L~170nm) Reducing stray serial resistance. Stray Capacitance area 1X0.5mm. VDS=10V VGS=-10V  10V Better than 100kHz BW.

38. Summary In degenerate semiconductors part of the current is due to energy flow (similar to the case of energy flow due to thermal gradient) It is possible to have some level of “post-synthesis” manipulation of materials with the aim to provide “building blocks” for device engineering Reducing the OLED’s micro-cavity emission to a simple expression allows for physical insight but more importantly – provides an engineering too. Vertical Field Effect Transistors – low voltage , high current & fast

39. In Collaboration with: Funding Uri Banin, AsafAharoni Hebrew Univ., Jerusalem NCs Israel Science Foundation Israel Ministry of Science Minerva Center YoavEichen, Shai Tal Technion Peptides Jung Il Jin, Yang Jun Yu, Korea Univ, Wide gap x-link Antonio Facchetti, Zhihua Chen, Polyera Corporation Ntype (VOFET) Thank You

40. Thomson effect,  the evolution or absorption of heat when electric current passes through a circuit composed of a single material that has a temperature difference along its length. This transfer of heat is superimposed on the common production of heat associated with the electrical resistance to currents in conductors. If a copper wire carrying a steady electric current is subjected to external heating at a short section while the rest remains cooler, heat is absorbed from the copper as the conventional current approaches the hot point, and heat is transferred to the copper just beyond the hot point. This effect was discovered (1854) by the British physicist William Thomson (Lord Kelvin) When carriers move from cold to hot they need to gain energy in order to be distributed in energy according to the new temperature. For that they absorb phonons (heat) and the conductor cools.